The field of the invention is that of electrical circuit systems and the invention relates more particularly to novel and improved substrates for use in mounting semiconductor devices and other circuit components in the electrical circuit systems.
Currently used or proposed insulated metal substrates for electronic circuits and the like embody a rigid, thermally conducting, metal base layer, a dielectric layer of polymer or ceramic or the like on the base layer, and a conductive copper film on the dielectric layer to be etched for forcing a desired circuit path pattern. Such substrates mount semiconductor devices and other circuit components in electrically and thermally connected relation to the circuit paths for forming circuit systems, and heat generated during operation of such systems is withdrawn from the semiconductor devices and other components and dissipated through the base layer of the substrates. In such substrates, a polymer dielectric layer is sometimes relied upon for bonding the electrically conductive circuit layer of the substrate to the base layer, and in other known substrates a film of dielectric material or the like is coated on both sides with an adhesive for bonding to the electrically conductive circuit layer and to the base layer. In such substrates the thickness of the dielectric layer is selected to provide the necessary degree of electrical insulation between the electrically conductive circuit layer and the heat-dissipating metal base layer to meet the requirements of specific electrical circuit systems. However it is frequently found that the thickness of the dielectric layer needed for this purpose tends to interfere to an undesirable like to the heat-dissipating base layer so that the semiconductor devices cannot be operated in their optimum temperature condition. As a result, some of the recently proposed substrates which incorporate polymer dielectric layers have the polymers filled with alpha alumina or other thermally conducting but electrically non-conducting materials for improving heat-transfer through the dielectric layer. Such dielectric layers tend to be difficult to form in very thin layers and, when provided in thin layers, are sometimes found to have non-uniform dispersal of the alumina therein so they provide non-uniform heat-withdrawal from various parts of an electrical circuit system. It has also been proposed from time to time that the surfaces of an aluminum heat sink member be anodized for forming an electrically insulating coating thereon but such systems also tend to require dielectric layers of substantial thickness for meeting electrical insulation requirements. More recently, electrical insulation and heat-withdrawal requirements for electrical circuit systems have tended to become progressively more stringent and it is found that known insulated metal substrates frequently do not provide suitably economical means for mounting semiconductor devices and the like while also providing both the electrical insulation and heat-dissipating properties required for such systems. It would be very desirable to provide a circuit system having an economical insulated metal substrate incorporating a dielectric layer of improved electrically insulating and thermally conducting properties.